This invention relates to an electroplating current supply system for electroplating an object to be plated, in which the polarity of current to be supplied to a load (hereinafter referred to as plating load) including the object, an electrolytic solution and electrodes, is inverted at a high speed. More particularly, this invention relates to such electroplating current supply system capable of uniformly plating a printed circuit board with through-holes and via-holes formed therein.
It is known to invert, at a high speed, the polarity of current supplied to a plating load. When current with the positive polarity is being supplied to the plating load, plating takes place, and when the current of the negative polarity is being supplied to the load, the plating is interrupted or part of the metal of the plated layer is dissolved back into the electrolyte solution, whereby crystals forming the plated layer are made finer so that the object can be uniformly plated.
When a multi-layered printed circuit board, like the one shown in FIG. 1, including substrates P1, P2, . . . , on which electronic components are integrated to a high density, is plated, the thickness of a plated metal layer M on an edge 11 of a through-hole 1 becomes different to a great extent from the thickness on an inner wall 12, as the number of the substrates of the printed circuit board increases. In other words, it is difficult to uniformly plate the printed circuit board as the number of the substrates forming the board increases. The same problem arises with respect to a via-hole 2, so that the thickness of the plated layer M on an edge 21 and the thickness of the layer M on the inner wall 22 become different from each other. It has been found that in order to form a plated metal layer M having a thickness which is uniform over the entire surfaces of the substrates P1, P2, . . . , it is necessary to make a negative-polarity plating current of sufficiently larger magnitude to flow for a shorter time period than a positive-polarity plating current, through the through-hole 1 and the via-hole 2.
In Japanese Patent Application No. HEI 10-281954 on Sep. 17, 1998 (Japanese Patent Application Publication No. 2000-92841), inventors including one of the inventors of the present invention proposed an electroplating current supply apparatus which supplies current having a polarity inverted at intervals of, for example, from 5 to 20 milliseconds, to a plating load, to thereby form a layer of uniform thickness over a multi-layered printed circuit board.
When the electroplating current supply apparatus proposed in the above-mentioned Japanese patent application or any other prior art electroplating current supply apparatuses is used to plate a multi-layered printed circuit board like the one shown in FIG. 1 with through-holes and via-holes formed therein, an electroplating current is usually supplied to respective constituent parts of the printed circuit board. It has been found that with such prior art current supply apparatuses, it is difficult to provide a plated layer having a uniform thickness over the entire board. Also, the resulting plated layer is not glossy.
An object of the present invention is to provide an electroplating current supply apparatus with which a multi-layered printed circuit board with through-holes and via-holes formed therein can be plated with a layer of a uniform thickness.
According to one embodiment of the present invention, an electroplating current supply system has a power supply unit including a DC power supply apparatus supplying a positive current and a DC power supply apparatus supplying a negative current. The positive and negative current supplying DC power supply apparatuses operate alternately with each other so that the power supply unit can supply a plating load with an electroplating current having a polarity inverted at predetermined intervals. The electroplating current supply system further includes a processing unit which controls the ratio between the positive current value of the electroplating current supplied to the plating load from the positive current supplying DC power supply apparatus and the negative current value of the electroplating current supplied to that plating load from the negative current supplying DC power supply apparatus, and also the ratio between the period during which the positive current is supplied to the plating load and the period during which the negative current is supplied to the plating load.
With the electroplating current supply system arranged as described above, a plated layer or coating having a uniform and optimum thickness can be formed on an object by properly controlling, by means of the processing unit, the ratios in magnitude and period of the positive current to the negative current supplied to the plating load.
The ratio in magnitude of the positive current to the negative current to be supplied to the plating load may be selected to be within a range of from 1:2 to 1:3.
The ratio in period of the positive current to the negative current to be supplied to the plating load may be selected to be within a range of from 10:1 to 30:1.
The ratio in magnitude of the positive current to the negative current to be supplied to the plating load may be selected to be within a range of from 1:2 to 1:3, with the ratio in period of the positive current to the negative current selected to be within a range of from 10:1 to 30:1.
According to another embodiment of the invention, an electroplating current supply system has a plurality of current supply units each including a DC power supply apparatus supplying a positive current and a DC power supply apparatus supplying a negative current. The positive and negative current supplying DC power supply apparatuses operate alternately with each other so that each current supply unit can supply an associated plating load with an electroplating current having a polarity inverted at predetermined intervals. The electroplating current supply system further includes a processing unit which controls the ratio between the positive current value of the electroplating current supplied to each plating load from the associated positive current supplying DC power supply apparatus and the negative current value of the electroplating current supplied to that plating load from the associated negative current supplying DC power supply apparatus, and also the ratio between the periods during which the positive and negative currents are supplied to each plating load.
By individually controlling the ratios in magnitude and period of the positive current to the negative current supplied to the respective plating loads, a plated layer having a thickness which is uniform and optimum for each of objects to be plated can be formed on that object.
The ratio in magnitude of the positive current to the negative current to be supplied to each plating load may be selected to be within a range of from 1:2 to 1:3.
The ratio in period of the positive current to the negative current to be supplied to each plating load may be selected to be within a range of from 10:1 to 30:1.
The ratio in magnitude of the positive current to the negative current to be supplied to each plating load may be selected to be within a range of from 1:2 to 1:3, with the ratio in period of the positive current to the negative current selected to be within a range of from 10:1 to 30:1.
The positive current supplying DC power supply apparatus may include a first DC power supply. Between one of the output terminals of the first DC power supply and one of load terminals to which the plating load is to be connected, connected is a series combination of a first reactor and a first main switching device, which switching device is turned on and off in response to a control signal supplied thereto from the processing unit. A first auxiliary switching device is connected between the one and other output terminals of the first DC power supply. The first auxiliary switching device is turned on and off in response to a control signal from the processing unit, in a manner complementary to the manner in which the first main switching device is turned off and on. The other output terminal of the first DC power supply is connected to the other one of the load terminals to which the plating load is to be connected.
The negative current supplying DC power supply apparatus includes a second DC power supply. Between one of the output terminals of the second DC power supply and the other load terminal to which the plating load is to be connected, connected is a series combination of a second reactor and a second main switching device, which switching device is turned on and off in response to the control signal supplied thereto from the processing unit. A second auxiliary switching device is connected between the one and other output terminals of the second DC power supply. The second auxiliary switching device is turned off and on in response to the control signal from the processing unit, in a manner complementary to the manner in which the second main switching device is turned on and off. The other output terminal of the second DC power supply is connected to the one load terminal to which the plating load is to be connected.